As one of the future alternative technologies of flash memory, resistive memories have drawn widespread attention due to their such characteristics as being high in density, low in cost, and being able to break through the restricts in development of process technology generations, etc. The resistive memory enables storage medium to be reversibly converted between a high resistance state (HRS) and a low resistance state (LRS) under the effect of electrical signals, thus realizing storage function. As one of the storage medium materials, metal oxide semi-conductor material is used by the resistive memory, such as copper oxide (CuxO, 1<x≦2),) tungsten oxide (WOx, 1<x≦3), titanium oxide, etc.
There are substantially two methods of fabricating the metal oxide: the first one is to form a layer of metal oxide as storage dielectric layer on the lower electrode by film deposition; the second one is to use this metal as lower electrode and then perform self-aligned oxidizing the lower electrode to form a layer of metal oxide as storage dielectric layer; wherein the second method is being widely used due to such characteristics as self-alignment and simplicity in technical process, etc.
In the second method, the storage characteristic of storage dielectric layer formed by oxidization is largely affected by evenness of the metal layer. The better the evenness of the metal layer is, the more consistent the characteristic of the metal oxide layer thus formed will be, and the better the consistency of the resistive memory will be.
However, during the formation of the lower electrode, the metal layer is generally formed in the dielectric layer by patterning, and when the metal layer is being formed in the dielectric layer, it is generally required to firstly form a diffusion barrier layer before forming the metal layer. Typically, the characteristic of metal layer in the middle area of metal layer differs much from that of metal layer adjacent to the border area with the dielectric layer. For example, the sizes of grains are different and the crystal orientations are different. When the metal oxide layer is formed by self-alignment oxidization of the metal layer, the characteristics of storage medium layers formed by oxidization will also differ greatly since the characteristics of metal layers in the same pattern differ much from each other. In particular, as the size is scaled down, it is difficult for each memory unit to ensure that the storage medium layer is formed by self-aligning with the middle area of the metal layer; therefore, it becomes more difficult for many memory units to ensure that the storage medium layer is each formed by self-aligning with the middle area of the metal layer. Thus, when this method is used to form resistive memory, a challenge is put to the consistency of memory.
The above problem will be discussed hereinafter with reference to a CuxO resistive memory.
FIG. 1 shows a schematic structural view of a CuxO resistive memory in the prior art, wherein a top view and a C-C sectional view of the resistive memory are shown. In the prior art, a CuxO resistive memory is generally integrated into a back-end structure of copper interconnection. That is, a copper lead or copper plug in the copper interconnect structure is used as a lower electrode and the storage medium layer is further formed by oxidization. As shown in FIG. 1, an inter-layer dielectric layer for forming a certain layer of copper lead is indicated by “10”, an etch-terminating layer is indicated by “11”, a cap layer is indicated by “13”, a diffusion barrier layer of the copper lead is indicated by “21”, a seed crystal layer in the copper lead is indicated by “22”, and a copper lead in the middle area is indicated by “23”. The copper lead in the middle area 23 (i.e. the middle area copper lead) and the seed crystal layer 22 situating in the edge area of the copper lead (i.e., the border area with the dielectric layer) generally exhibit distinctly different characteristics; the sizes and crystal orientations of grains are different. When oxidization is performed on the copper lead, the rate of oxidization will differ between the middle area and the edge area, and the storage characteristic of the CuxO storage medium layer 30 formed by oxidization would also be uneven. When the oxidization is performed, generally, a hole is provided in the cap layer 12 so as to expose the copper lead and perform oxidization. However, since the feature size is becoming smaller and smaller, the width of the copper lead itself (in the left-right direction shown in the drawing) is also becoming smaller and smaller. If only the middle area 23 is exposed to perform patterning and oxidization, it is necessary to set the size of hole in the cap layer 12 to be very small, which would greatly increase cost of process; moreover, during oxidization, the oxidized film will also partially grow diffusely towards two sides; therefore, if the metal layer is not exposed at a smaller size and with concentration in the middle area, the performances of the CuxO storage medium layer 30 formed by the seed crystal layer 22 (the areas circled by dashed-lines in FIG. 1) and the CuxO storage medium layer 30 formed in the middle area 23 are different for the same memory unit; for multiple memory units, the CuxO storage medium layer of some memory units may be formed in the middle area 23, and the CuxO storage medium layer of some memory units may be also formed by the seed crystal layer 22; thus, there arises problem with this type of resistive memory in terms of consistency.
Similarly, the above problem also exists with other electrode materials (e.g., tungsten) formed in the dielectric layer by patterning.